1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, an electrode unit in which a positive electrode plate, a negative electrode plate and a separator interposed therebetween, are wound, and a secondary battery using the same.
2. Description of the Related Art
With rapid advancement of lightweight, wireless electronic devices such as mobile phones, camcorders, notebook type computers and the like, development of lithium ion batteries having a high-energy density power supply of such electric devices is being actively conducted. A secondary battery is formed by accommodating a circular or noncircular spiral electrode unit in a circular or rectangular can or pouch.
In such a secondary battery, an electrode unit is spirally wound wherein a positive electrode strip is coated with a positive electrode active material, except a portion of either end of a strip-like positive electrode collector (to be referred to as a positive electrode uncoated portion), a negative electrode strip is coated with a negative electrode active material, except a portion of either end of a strip-like negative electrode collector (to be referred to as a negative electrode uncoated portion), and a separator is interposed between the positive electrode strip and the negative electrode strip. A lead is installed at either the positive electrode uncoated portion or the negative electrode uncoated portion.
Since the uncoated portions are formed of thin films, sliding may occur between the negative and positive electrode uncoated portions during an initial winding stage. Leads are installed at the positive and negative electrode uncoated portions, and, if necessary, a tape is adhered to portions to which the leads are connected, to ensure insulating efficiency between the positive and negative electrode uncoated portions. However, if a burr occurs on at least one side of the positive and negative electrode uncoated portions, the insulating efficiency may be lowered. Also, even if the wound positive and negative electrode uncoated portions are insulated from each other by the separator, the insulating reliability is relatively poor.
Japanese Patent Publication No. Hei 8-339817 discloses an electrode unit of a non-circular, spiral battery. In the disclosed electrode unit, the winding start portions of the positive and the negative electrode plates are deviated, only the first wound electrode plate is positioned at a folded portion of the innermost electrode plate, and a winding start portion of a later wound electrode plate lags behind the first folded portion of the first wound electrode plate. The first folded portion of the first wound electrode plate is opposite to the first wound electrode plate around the periphery of the electrode unit.
U.S. Pat. No. 5,508,122 discloses a battery having a spiral electrode unit. The disclosed electrode unit is configured such that the same polarities are positioned at both sides of exposed areas of a negative electrode strip made of a core material and separated via a separator.
Japanese Patent Publication No. Hei 11-111327 discloses a battery having a spiral electrode unit in which a negative electrode plate is positioned at the innermost winding part of the electrode unit, and a positive electrode plate made of a copper foil is positioned at the outermost winding part.
As described above, in the conventional spiral electrode units, although exposed regions of positive and negative electrodes, that is, uncoated portions, are described, the problem of deviated winding is still unresolved. Also, since there is no impact absorbing member in the central portion of the electrode unit, the electrode unit is prone to external impacts.